Dice your chicken, celery, onion, and apple and add to a large mixing bowl.
In a smaller mixing bowl, add the mayonnaise, yogurt, mustard, salt, and pepper. Mix well.
Pour the dressing in the large bowl with the chicken, celery, onion, and apple. Mix well.
Add more salt and pepper to taste.
Serve on a croissant, bagel, crackers, or toast with a slice of cheese.